The intense commercial interest in ‘three-dimensional’ or stereoscopic motion imaging—particularly television—has led to the desire to convert existing ‘2D’ image streams to ‘3D’. This is because the creation of stereoscopic material is complicated and expensive and so it is difficult to obtain sufficient content to sustain a 3D broadcast service or ‘channel’. There are a number of known methods for deriving left-eye and right-eye images for stereoscopic display from a common, 2D input image in such a way that a ‘synthetic’ 3D image is obtained. In these methods horizontal disparity (positional shift) between the left-eye and right-eye images is introduced over all or part of the image, so that binocular vision provides the illusion that the image, or image region, is either in front of or behind the plane of a stereoscopic image display.
Some conversion methods identify objects or regions of interest by suitable image segmentation techniques, and then introduce horizontal disparity between the left-eye and right-eye representations of those objects or regions. However, image segmentation is difficult, and it is also hard to determine the required disparity that gives a convincing 3D effect.
There is a simpler, and widely-used, method of creating synthetic 3D that is applicable to horizontally-moving images; this is to introduce a temporal offset between the left-eye and right-eye images. The temporal offset results in a spatial offset (i.e. a disparity) equal to the product of the motion speed, in pixels per frame period, and the temporal offset, in frame periods. For example, if the camera pans to follow a horizontally moving object, the background will move, and the followed object will not move, so that disparity will be added only to the background. Provided that the direction of the temporal offset is controlled by the direction of pan, the background will appear further away from the viewer than the followed object. If this technique is combined with the addition of a ‘global’ (i.e. constant over the whole image area), and/or spatially-determined (i.e. dependent on position within the image frame) disparity, then a quite realistic illusion of depth can be obtained.
Commercial broadcasters are concerned to ensure that 3D is recognised as a ‘premium’ service, justifying high subscription payments by viewers and high charges for associated advertising. And, public-service broadcasters want to maintain their reputations as providers of high-quality programme material. For these reasons, technical specifications for the commissioning of video material often include restrictions on the use of synthetic 3D, because of doubts about its subjective quality. There is thus a need for the detection of synthetic 3D as part of ‘quality assurance’ systems used by organisations that commission video content from other organisations.